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Real Claude CLI traffic sends cache_control as
`{"type":"ephemeral","ttl":"1h"}`. Our previous payload only
sent `{"type":"ephemeral"}`, which is a bytewise mismatch with
the official CLI and one more third-party detection signal.

Policy: client-provided ttl is always passed through unchanged.
Proxy-generated cache_control blocks default to 5m (vs Parrot's 1h)
to avoid burning the 1h cache budget on automatic breakpoints while
still aligning with the `ttl` field being present.

  - claude/constants.go: DefaultCacheControlTTL = "5m"
  - apicompat/types.go: new AnthropicCacheControl type with TTL field;
    AnthropicTool gains optional CacheControl pointer so the mimicry
    path can attach a cache breakpoint to tools[-1] later.
  - service/gateway_service.go: anthropicCacheControlPayload gains TTL;
    marshalAnthropicSystemTextBlock and rewriteSystemForNonClaudeCode
    emit ttl=5m by default.